Methods for selectively separating a certain gas from a gas mixture using a membrane are well known. Since a solution-diffusion model for gas separation membranes was proposed by T. Graham in the mid-1800s, research on gas separation membranes has been actively conducted, and the gas separation membranes has been commercially available and applied to various fields by the 1980s.
Generally, a gas permeation phenomenon in which gases pass through a non-porous membrane is explained using the solution-diffusion model proposed by T. Graham. In this case, respective gas components appear to have different relative diffusion coefficients and solubility constants according to materials constituting a membrane. Also, the most preferred materials for gas separation membranes are known to exhibit high permeability and high selectivity at the same time. Robeson (1991, J. Membr, Sci, 62, 165) found that, since there is a correlation between permeability and selectivity, the selectivity decreases when the permeability increases, whereas the permeability decreases when the selectivity increases, and thus proposed an upper limit in the correlation between the permeability and the selectivity. The fact that it is difficult to develop membrane materials exceeding the upper limit proposed by Robeson and it is far difficult to commercialize the membrane materials is known as one issue.
Therefore, methods of changing the conditions for a gas separation process, such as pressure difference, compositions, temperature and the like, have been used instead of developing a separation membrane exhibiting high permeability and high selectivity at the same time. However, since changing the conditions is changing process conditions, an increase in costs may be caused.
Meanwhile, in a process of producing an expandable polystyrene (hereinafter referred to as an “EPS”), an excessive amount of pentane that is a volatile organic compound (VOC) is introduced as a foaming agent into an EPS reactor so that the pentane is impregnated into polystyrene (PS) beads, and unimpregnated pentane is discharged out from the reactor. To prevent such discharged pentane from being discharged into the air, the discharged pentane is, for example, incinerated in a regenerative thermal oxidation (RTO) system, and then discharged into the air. In this process, however, a large amount of toxic gases such as carbon monoxide, carbon dioxide, or nitrogen oxides may be discharged into the air.
Since approximately 5 to 20% of the added foaming agent may be lost in this process, the manufacturing cost of the polystyrene may increase in an aspect of economic efficiency.